1. Field of the Invention
The present invention relates to improvements in the use of quantum dot energy as photosensibilizers to create singlet oxygen from inert nanoparticles. More particularly, the invention relates to improvements particularly suited for effecting cell apoptosis causing natural cell death by generating singlet oxygen from inert nanoparticles having a silicon core with a thin oxygen and/or nitrogen shell capable of being activated to stimulate formation of said singlet oxygen radicals with a normal low cost visible light source generator. In particular, the present invention relates specifically to a silicon nanoparticle with a 0.5 to 1.5 nm shell.
2. Description of the Known Art
As will be appreciated by those skilled in the art, various types of quantum dots are known in various forms. Similarly, various photodynamic cancer treatments are known than use photo sensibilizers. What is not know is the use of an inert quantum dot capable of use with low power common light sources for inexpensive cancer therapy. Patents applications disclosing information relevant to quantum dots include U.S. provisional application Ser. No. 60/730,271 filed on Oct. 26, 2005; U.S. application Ser. No. 11/094,837 filed on Mar. 30, 2005; and U.S. provisional application Ser. No. 60/558,209 filed on Mar. 30, 2004. Each of these applications is hereby expressly incorporated by reference in their entirety.
As will be appreciated by those skilled in the art, silicon nanoparticles are known in various forms. Patents disclosing information relevant to silicon nanoparticles include U.S. Pat. No. 7,078,276, issued to Zurcher, et al. on Jul. 18, 2006; U.S. Pat. No. 7,020,372, issued to Lee, et al. on Mar. 28, 2006; U.S. Pat. No. 7,005,669, issued to Lee on Feb. 28, 2006; U.S. Pat. No. 6,961,499, issued to Lee, et al. on Nov. 1, 2005; U.S. Pat. No. 6,846,565, issued to Korgel, et al. on Jan. 25, 2005; and U.S. Pat. No. 6,268,041, issued to Goldstein on Jul. 31, 2001; U.S. Pat. No. 6,992,298, issued to Nayfeh, et al. on Jan. 31, 2006. Each of these patents is hereby expressly incorporated by reference in their entirety.
Other publications to consider include: 1. C. Delerue, G. Allan, M. Lannoo, Optical band gap of Si nanoclusters, J. Lumin. 1999, v. 80, pp. 65-73; 2. Y. D. Glinka, Size effect in self-trapped exciton photoluminescence from SiO2-based nanoscale materials, Physical Review B., 2001, v. 64, p 085421; 3. S. Altman, D. Lee, J. D. Chung, J. Song, M. Choi, Light absorption of silica nanoparticles, Phys. Rev. B., 2001, v. 63, p. 161402; 4. L. Brus, Electronic Wave Functions in Semiconductor Clusters: Experiment and Theory, J. Phys. Chem., 1986, v. 90, pp. 2555-2560; 5. E. A. Konstantinova, V. A. Demin, A. S. Vorontsov, Yu. V. Ryabchikov, I. A. Belogorokhov, L. A. Osminkina, P. A. Forsh, P. K. Kashkarov, V. Yu. Timoshenko, Electron Paramagnetic Resonance and Photoluminescence Study of Si Nanocrystals-Photosensitizers of Singlet Oxygen Molecules, J. Non-Cryst. Sol., 2006, v. 352, pp. 1156-1159; 6. N. J. Turro, Modern Molecular Photochemistry, University Science Publications, Sausalito, Calif., 1991; 7. Kuz'min G. P., Karasev M. E., Khokhlov E. M., Kononov N. N., Korovin S. B., Plotnichenko V. G., Polyakov S. N., V. I. P., O. V. T. Nanosize Silicon Powders: The Structure and Optical Properties//Laser Phys.—2000.—V. 10.—No. 4.—P. 939-945; 8. A. A. Ischenko, A. A. Sviridova, K. V. Zaitseva, O. A. Rybaltovsky, V. N. Bagratashvili, A. I. Belogorokhov, V. V. Koltashev, V. G. Plotnichenko, I. A. Tutorsky, Spectral properties of siliceous nanocomposite materials. Proc. SPIE, 2006, v. 6164, pp. 616406-1-616406-7; 9. A. O. Rybaltovsky, V. A, Radzig, A. A. Sviridova, A. A. Ischenko, Effect of annealing on the Silicon Nanocrystals optical properties, Nanotechnic, 2007, v.13(11), pp.116-121; and 10. W. Kueng, E. Silber, and U. Eppenberger, Annals of Biochemistry, 1989, v.182, pp.16-21. Each of these patents and/or publications is hereby expressly incorporated by reference in their entirety. As noted by these disclosures, the prior art is very limited in its teaching and utilization, and an improved nanocrystaline based therapy is needed to overcome these limitations.
The present invention is addressed to a previously undiscovered method for generating singlet oxygen for use in photodynamic therapy. Several issues need to be considered to understand the background of the present invention, including photodynamic therapy, singlet oxygen, excitons, and limitations of the prior art teachings.
Chemical Based Photodynamic Therapy
The following basic background information provided in paragraphs (a) through (e) was presented by the U.S. National cancer institute in describing the old methods for Photodynamic therapy (PDT):
(a) PDT is a treatment that uses a drug (chemical), called a photosensitizer or photosensitizing agent, and a particular type of light. When photosensitizers are exposed to a specific wavelength of light, they produce an activated form of oxygen that kills nearby cells. Each photosensitizer is activated by light of a specific wavelength. This wavelength determines how far the light can travel into the body. Thus, doctors use specific photosensitizers and wavelengths of light to treat different areas of the body with PDT. In the first step of PDT for cancer treatment, a photosensitizing agent is injected into the bloodstream. The agent is absorbed by cells all over the body, but stays in cancer cells longer than it does in normal cells. Approximately 24 to 72 hours after injection, when most of the agent has left normal cells but remains in cancer cells, the tumor is exposed to light. The photosensitizer chemical in the tumor absorbs the light and produces an active form of oxygen that destroys nearby cancer cells by killing them (necrosis) rather than by the natural cell death mechanism (apoptosis). In addition to directly killing cancer cells, PDT appears to shrink or destroy tumors in two other ways. The photosensitizer can damage blood vessels in the tumor, thereby preventing the cancer from receiving necessary nutrients. In addition, PDT may activate the immune system to attack the tumor cells.
(b) The light used for PDT can come from a laser or other sources of light. Laser light can be directed through fiber optic cables (thin fibers that transmit light) to deliver light to areas inside the body. For example, fiber optic cable can be inserted through an endoscope (a thin, lighted tube used to look at tissues inside the body) into the lungs or esophagus to treat cancer in these organs. Other light sources include light-emitting diodes (LEDs), which may be used for surface tumors, such as skin cancer. PDT is usually performed as an outpatient procedure. PDT may also be repeated and may be used with other therapies, such as surgery, radiation, or chemotherapy.
(c) To date, the U.S. Food and Drug Administration (FDA) has approved the photosensitizing agent called porfimer sodium, or PHOTOFRIN®, for use in PDT to treat or relieve the symptoms of esophageal cancer and non-small cell lung cancer. Porfimer sodium is approved to relieve symptoms of esophageal cancer when the cancer obstructs the esophagus or when the cancer cannot be satisfactorily treated with laser therapy alone. Porfimer sodium is used to treat non-small cell lung cancer in patients for whom the usual treatments are not appropriate, and to relieve symptoms in patients with non-small cell lung cancer that obstructs the airways. In 2003, the FDA approved porfimer sodium for the treatment of precancerous lesions in patients with Barrett's esophagus (a condition that can lead to esophageal cancer). Porfimer sodium makes the skin and eyes sensitive to light for approximately 6 weeks after treatment. Thus, patients are advised to avoid direct sunlight and bright indoor light for at least 6 weeks. Photosensitizers tend to build up in tumors and the activating light is focused on the tumor. As a result, damage to healthy tissue is minimal. However, PDT can cause burns, swelling, pain, and scarring in nearby healthy tissue. Other side effects of PDT are related to the area that is treated. They can include coughing, trouble swallowing, stomach pain, painful breathing, or shortness of breath; these side effects are usually temporary.
(d) The light needed to activate most photosensitizers cannot pass through more than about one-third of an inch of tissue (1 centimeter). For this reason, PDT is usually used to treat tumors on or just under the skin or on the lining of internal organs or cavities. PDT is also less effective in treating large tumors, because the light cannot pass far into these tumors. PDT is a local treatment and generally cannot be used to treat cancer that has spread (metastasized).
(e) Researchers continue to study ways to improve the effectiveness of PDT and expand it to other cancers. Clinical trials (research studies) are under way to evaluate the use of PDT for cancers of the brain, skin, prostate, cervix, and peritoneal cavity (the space in the abdomen that contains the intestines, stomachs and liver). Other research is focused on the development of photosensitizers that are more powerful, more specifically target cancer cells, and are activated by light that can penetrate tissue and treat deep or large tumors. Researchers are also investigating ways to improve equipment and the delivery of the activating light.
As noted by this basic information, several problems exist with current photosensibilizers due to patient sensitivity increases for up to 6 week periods, overexposure of the patient to the photosensibilizers, the expense and difficulty associated with this class of photosensibilizers, and most importantly the difficulty of precise delivery of the singlet oxygen to specific tumor cells by having to “shoot beams of light” onto targets of photosensitizing agents administered by system injection to all body cells. Thus, an improved photosensibilizer for the generation of a singlet oxygen is needed along with an improved and precise method of application and treatment delivery.
Apoptosis
Apoptosis (pronounced {hacek over (a)}-põp-tõ's{hacek over (i)}s[1]) is a form of programmed cell death in multicellular organisms. It is the primary method of programmed cell death (PCD) that allows body organs to remain of similar size throughout adult life even as cells replace themselves continually in the normal life process. It involves a series of biochemical events leading to a characteristic cell morphology and death, in more specific terms, a series of biochemical events that lead to a variety of morphological changes, including blebbing, changes to the cell membrane such as loss of membrane asymmetry and attachment, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation. Processes of disposal of cellular debris whose results do not damage the organism differentiates apoptosis from necrosis.
In contrast to necrosis, which is a form of traumatic cell death that results from acute cellular injury, apoptosis, in general, confers advantages during an organism's life cycle. Between 50 billion and 70 billion cells die each day due to apoptosis in the average human adult. For an average child between the ages of 8 and 14, approximately 20 billion to 30 billion cells die a day. In a year, this amounts to the proliferation and subsequent destruction of a mass of cells equal to an individual's body weight.
Research on apoptosis has increased substantially since the early 1990s. In addition to its importance as a biological phenomenon, defective apoptotic processes have been implicated in an extensive variety of diseases. Excessive apoptosis causes hypotrophy, such as in ischemic damage, whereas an insufficient amount results in uncontrolled cell proliferation, such as cancer.
Apoptosis can occur when a cell is damaged beyond repair, infected with a virus, or undergoing stress conditions such as starvation. DNA damage from ionizing radiation or toxic chemicals can also induce apoptosis via the actions of the tumour-suppressing gene. The “decision” for apoptosis can come from the cell itself, from the surrounding tissue, or from a cell that is part of the immune system. In these cases apoptosis functions to remove the damaged cell, preventing it from sapping further nutrients from the organism, or to prevent the spread of viral infection.
The process of apoptosis is controlled by a diverse range of cell signals, which may originate either extracellularly (extrinsic inducers) or intracellularly (intrinsic inducers). Extracellular signals may include hormones, growth factors, nitric oxide or cytokines, and therefore must either cross the plasma membrane or transduce to effect a response. These signals may positively or negatively induce apoptosis; in this context the binding and subsequent initiation of apoptosis by a molecule is termed positive, whereas the active repression of apoptosis by a molecule is termed negative.
Dying cells that undergo the final stages of apoptosis display phagocytotic molecules, such as phosphatidylserine, on their cell surface. Phosphatidylserine is normally found on the cytosolic surface of the plasma membrane, but is redistributed during apoptosis to the extracellular surface by a hypothetical protein known as scramblase. These molecules mark the cell for phagocytosis by cells possessing the appropriate receptors, such as macrophages. Upon recognition, the phagocyte reorganizes its cytoskeleton for engulfment of the cell. The removal of dying cells by phagocytes occurs in an orderly manner without eliciting an inflammatory response.
Singlet Oxygen
Singlet oxygen is the common name used for the two metastable states of molecular oxygen (O2) with higher energy than the ground state triplet oxygen. The energy difference between the lowest energy of O2 in the singlet state and the lowest energy in the triplet state is about 3625 kelvin (Te(a1Δg<−X3Σg−)=7918.1 cm−1.) Molecular oxygen differs from most molecules in having an open-shell triplet ground state, O2(X3Σg−). Molecular orbital theory predicts two low-lying excited singlet states O2(a1Δg) and O2(b1Σg+). These electronic states differ only in the spin and the occupancy of oxygen's two degenerate antibonding πg-orbitals. The O2(b1Σg+)-state is very short lived and relaxes quickly to the lowest lying excited state, O2(a1Δg). Thus, the O2(a1Δg)-state is commonly referred to as singlet oxygen.
The energy difference between ground state and singlet oxygen is 94.2 kJ/mol and corresponds to a transition in the near-infrared at ˜1270 nm. In the isolated molecule, the transition is strictly forbidden by spin, symmetry and parity selection rules, making it one of nature's most forbidden transitions. In other words, direct excitation of ground state oxygen by light to form singlet oxygen is very improbable. As a consequence, singlet oxygen in the gas phase is extremely long lived (72 minutes). Interaction with solvents, however, reduces the lifetime to microsecond or even nanoseconds.
Formation of Singlet Oxygen
Formation of singlet oxygen is known using chemical reactions or the use light on dyes as shown in (WO/1997/029044) DEVICE FOR PRODUCING A SINGLET OXYGEN ACTIVATED GAS STREAM, August, 1997 which notes the following: Known equipment exists for the production of singlet oxygen and photo-sensitive means for this purpose. In “Singlet 02” by Aryeh A. Frimer, CRC Press Inc., USA 1985, the principles are described for production of singlet oxygen in a gaseous state, and thereby activated gas. WO patent application 9007144 indicates various photo-sensitive means for, in combination with light radiation, forming singlet oxygen which is employed for oxidation of specific compounds. WO patent application 9100241 concerns decomposition of nitrogen oxides. The decomposition is performed by the influence of light on a catalyst when a radiation source is placed against a transparent wall of a container. DE patent 4125254 describes a device for producing activated oxygen. The device which is described consists of a chamber in which through-flowing oxygen is irradiated from a UV radiation source and the chamber is divided by partitions into forward and backward flow paths, thus obtaining the longest possible flow path in order to achieve the longest possible treatment time for the oxygen. There is further described a finishing treatment with magnetic influence of the end product. However, the device is not intended for generating singlet oxygen, but for so-called “softer activation of the oxygen”. DE patent 3606925 describes a device for producing singlet oxygen and possibly ozone. The device is tubular, with a lamp in the middle and with a through-flow of oxygen, where a layer of metal oxides or a fluoridating material is provided on the surfaces of the device. The design of the device is extremely complicated. The known devices which have been employed for production of singlet oxygen have been large and cumbersome and/or complicated or it has not been possible to document that the production of singlet oxygen has actually taken place.
Sensibilizer
It is known that electrons are liberated when electromagnetic radiation, such as sun light, impinges on substances having a low ionization potential, so-called sensibilizers, whereby an electron-ion pair is formed.
Exciton
An exciton is a bound state of an electron and an imaginary particle called an electron hole in an insulator or semiconductor, and such is a Coulomb-correlated electron-hole pair. It is an elementary excitation, or a quasiparticle of a solid.
A vivid picture of exciton formation is as follows: a photon (particle of light energy) enters a semiconductor, exciting an electron from the valence band into the conduction band. The missing electron in the valence band leaves a hole behind, of opposite electric charge, to which it is attracted by the Coulomb force. The exciton results from the binding of the electron with its hole; as a result, the exciton has slightly less energy than the unbound electron and hole. The wave function of the bound state is hydrogenic (an “exotic atom” state akin to that of a hydrogen atom). However, the binding energy is much smaller and the size much bigger than a hydrogen atom because of the effects of screening and the effective mass of the constituents in the material.
Silicon Based Nanocrystals
A nanocrystal is a crystalline material with dimensions measured in nanometers; a nanoparticle with a structure that is mostly crystalline. These materials are of huge technological interest since many of their electrical, opto-electrical, and thermodynamic properties show strong size dependence and can therefore be controlled through careful manufacturing processes. Nanocrystal is part of the large “family” of nanotechnology. Semiconductor nanocrystals in the sub-10 nm size range are often referred to as nanoparticles.
Nanoparticles
A nanoparticle is defined by size alone. A nanostructure semiconductor is composed such that it confines the motion of conduction band electrons, valence band holes, or excitons (bound pairs of conduction band electrons and valence band holes) in all three spatial directions. The confinement can be due to electrostatic potentials (generated by external electrodes, doping, strain, impurities), the presence of an interface between different semiconductor materials (e.g. in core-shell nanocrystal systems), the presence of the semiconductor surface (e.g. semiconductor nanocrystal), or a combination of these. A quantum dot is a quantity of light/wave energy that has a discrete quantized amount of energy specific to the light spectrum. The corresponding wave functions are spatially localized within the particle, but extend over many periods of the crystal lattice. A quantum active nanoparticle contains a small finite number (of the order of 1-100) of conduction band electrons, valence band holes, or excitons, i.e., a finite number of elementary electric charges.
Small quantum active particles, such as colloidal semiconductor nanocrystals, can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a total of 100 to 100,000 atoms within the quantum active particle volume. Self-assembled quantum nanoparticles are typically between 10 and 50 nm in size. Nanoparticles defined by lithographically patterned gate electrodes, or by etching on two-dimensional electron gases in semiconductor heterostructures can have lateral dimensions exceeding 100 nm. At 10 nm in diameter, nearly 3 million nanoparticles could be lined up end to end and fit within the width of a human thumb (note: they cannot be used when lined up like this at the present).
The ability to tune the size of nanoparticles is advantageous for many applications. For instance, larger quantum active nanoparticles have spectra shifted towards the red compared to smaller dots, and exhibit less pronounced quantum properties. Conversely the smaller particles allow one to take advantage of quantum properties.
In large numbers, nanoparticles may be synthesized by means of a colloidal synthesis. Colloidal synthesis is by far the cheapest and has the advantage of being able to occur at benchtop conditions. It is acknowledged to be the least toxic of all the different forms of synthesis.
Nanoparticles may have the potential to increase the efficiency and reduce the cost of today's typical silicon photovoltaic cells. According to experimental proof from 2006, nanoparticles of lead selenide can produce as many as seven excitons from one high energy photon of sunlight (7.8 times the bandgap energy). Quantum dot nanoparticle photovoltaics would theoretically be cheaper to manufacture, as they can be made “using simple chemical reactions”.
Mercury Vapor Lamps
A mercury-vapor lamp is a gas discharge lamp which uses mercury in an excited state to produce light. The arc discharge is generally confined to a small fused quartz arc tube mounted within a larger borosilicate glass bulb. The outer bulb may be clear or coated with a phosphor; in either case, the outer bulb provides thermal insulation, protection from ultraviolet radiation, and a convenient mounting for the fused quartz arc tube. Mercury vapor lamps (and their relatives) are often used because they are relatively efficient. Phosphor coated bulbs offer better color rendition than either high- or low-pressure sodium vapor lamps. They also offer a very long lifetime, as well as intense lighting for several applications. A closely-related lamp design called the metal halide lamp uses various other elements in an amalgam with the mercury. Sodium iodide and Scandium iodide are commonly in use. These lamps can produce much better quality light without resorting to phosphors.
With all of this information in mind, it may be seen that these prior art teachings, publications, and patents are very limited in their teaching and utilization, and an improved silicon nanoparticle and method of use is needed to overcome these limitations.